PURPOSE: To enable accurate analysis to be conducted even if the intensity of primary X-rays is changed by processing the intensity of secondary X-rays by the intensity of primary X-rays, and thereby eliminating the effect of change in the intensity of primary X-rays.

CONSTITUTION: Primary X-rays from a X-ray source are diffracted by an artificial lattice 2, and formed into monochromatic light so as to be incident on a specimen 4 at an appropriate incident angle , and it is then detected by a detector (transmission type proportion counting tube) 3, so that its intensity is thereby detected. Secondary X-rays generated by the specimen 4 is then detected 6 so as to be detected 8 of its intensity, and a strength ratio with the primary X-rays is computed 9, so that a relative relation between the strength ratio and the incident angle is thereby formed 10. An adjuster 5 is so designed that the rotation angle of a specimen table 11 is set to be an incident angle most suitable to total reflection based on the strength ratio and the relative relation, and also that the change of the strength of the secondary X-rays with respect to the angle can be automatically detected 6. By this constitution, since the strength of the secondary X-rays is changed proportionally with the strength of the primary X-rays even if the latter is changed, a relation between a strength ratio and the concentration of the specimen 4 is not needed, so that correct concentration can thereby be detected at all times.